1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a recording image display device or a facsimile machine in which an electrostatic latent image formed on an image bearing member is developed to form a visible image by using the electrophotographic system, the electrostatic recording system or the like.
In particular, the present invention relates to an image forming apparatus including a charging device which abuts against an image bearing member, a magnetic brush portion of a magnetic brush charging member having a magnetic brush portion formed by magnetically binding charge magnetic carrier particles, and applies a charge bias to the magnetic brush charging member to charge the image bearing member, an image information writing device that forms an electrostatic latent image on the charged image bearing member, a developing device that visualizes the electrostatic latent image as a toner image by using a two-component developer consisting of toner particles and developing magnetic carrier particles, and a transferring device which transfers the toner image onto a transfer material, in which the developing device serves as cleaning means that collects the toner remaining on the image bearing member after the toner image has been transferred onto the transfer material by the transferring member.
2. Related Background Art
(A) Developing Device
In general, in a developing device (developing machine) equipped in an image forming apparatus of the electrophotographic system or the electrostatic recording system, there is used a two-component developer mainly containing toner particles and magnetic developing carrier particles (hereinafter referred to as xe2x80x9cdeveloping carriersxe2x80x9d). In particular, in most of color image forming apparatuses that form a full-color or multi-color image by the electrophotographic system, the two-component developer is employed from the viewpoint of the color tone etc. of an image.
As well known, the toner density of the two-component developer (that is, the ratio of the toner weight to the total weight of the developing carrier particles and the toner particles) is extremely important in stabilization of the image quality. The toner particles in the developer are consumed during a developing process, and the toner density of the developer is varied. For that reason, it is necessary that the toner density of the developer is accurately detected by using a developer density control device (ATR), a toner is replenished in accordance with a change in the density of toner, the density of toner is always controlled to a constant value (T/D ratio control) and the quality of image is maintained.
In order to correct the above change in the developer density within a developer container of the developing device which is caused by developing, that is, in order to control the quantity of toner which is replenished to the developer, devices for detecting and controlling the toner density of the developer within the developer container have been variously proposed and put into practical use.
For example, the following devices have been employed:
(a) a developer density control device disposed in the vicinity of a developer bearing member (called xe2x80x9cdeveloper sleevexe2x80x9d in the following description, since there are generally many cases in which a developer sleeve is used) or a developer carrying path in the developer container, for detecting and controlling the density of toner by utilizing the fact that a reflectance of a light applied to a developer carried onto the developer sleeve, or a developer within the developer container differs depending on the density of toner;
(b) a developer density control device of the inductance detecting system designed such that an inductance head (sensor) for detecting an apparent permeability caused by the mixture ratio of magnetic developing carriers and non-magnetic toner particles to convert it into an electric signal, is located on a side wall of the developing device, the actual toner density of the developer within the developer container is detected according to a detected signal from the inductance head, and the toner is replenished according to a comparison of the actual toner density with a reference value; and
(c) a developer density control device of the system in which a patch image density formed on an image bearing member (called xe2x80x9cphotosensitive drumxe2x80x9d in the following description, since there are generally many cases in which a photosensitive drum is used) is read by using a light source and a patch sensor for receiving a reflected light from a surface thereof, located opposite to the surface of the photosensitive drum, converted into a digital signal by an analog-to-digital convertor, and thereafter is transmitted to a CPU. In the CPU, the patch image density thus converted into the digital signal is compared with an initial set value, and if the density is higher than the initial set value, the replenishment of the toner is then suspended until the density is returned to the initial set value. On the other hand, if the density is lower than the initial set value, the toner is forcedly replenished until the density is returned to the initial set value, as a result of which the density of toner is indirectly maintained to a desired value.
However, the system described in the above item (a) in which the density of toner is detected according to the reflectance of a light applied to a developer carried onto the developer sleeve or a developer within the developer container, suffers from the following problem. That is, if the detecting means is stained by scattered toner or the like, the density of toner cannot be accurately detected. Also, the system described in the above item (c) in which the density of toner is indirectly controlled according to the patch image density suffers from a problem in that a space where a patch image is formed or a space where the detecting means is located cannot be ensured with a downsized copying machine or image forming apparatus.
On the contrary, the inductance detecting system described in the above item (b) is regarded as the optimum toner density detecting system in the copying machine or image forming apparatus which is low in the cost and small in space, because the system is not affected by the above described problem of the spaces and problem of the stain due to the scattered toner in addition that the cost of a sensor unit is low.
The above developer density control device of the inductance detecting system (inductance detecting system ATR) controls the density of toner according to the following manner. That is, control is made such that the replenishment of toner starts if it is detected that the apparent permeability of the developer is large, since it means that the ratio of the developing carrier to the developer within a constant volume becomes high and the density of toner becomes lower. On the contrary, the replenishment of toner is suspended if it is detected that the apparent permeability of the developer is small, since it means that the ratio of the developing carrier to the developer within the constant volume becomes lower and the density of toner becomes higher.
(B) Magnetic Brush Charging Device
In general, a corona photosensitive member has been employed as charging means for an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric equipped in the image forming apparatus of the electrophotographic system or the electrostatic recording system.
In recent years, because of having merits of low ozone, low electric power, etc., a contact charging device has been put into practical use, that is, a device of the system in which a charging member to which a voltage is applied is abutted against a member to be charged to allow the member to be charged. In particular, a device of the roller charging system using an electrically conductive roller as the charging member has been preferably employed from the viewpoint of the charging stability.
However, in the above-described roller charging system, because charging is conducted by discharging from the charging member to the member to be charged, the surface potential of the photosensitive member also fluctuates due to a fluctuation of the electric resistances of the charging roller and the electrophotographic photosensitive member, which is caused by a change in the environments.
Under the above circumstances, there has recently been proposed, as a charging system which is hardly influenced by a change in the environments, a method of conducting contact charging in such a manner that a voltage is applied to an electrically conductive contact charging member (a charging fur brush, a charging magnetic brush, a charging roller or the like) and charges identical in polarity with the potential of the photosensitive member are injected into the photosensitive member having on a surface thereof a charge injecting layer where electrically conductive powders (SnO2 or the like) having a trap potential are dispersed.
The injection charging system is advantageous in that the supply voltage is satisfactorily the same degree as the potential of the photosensitive member, and no ozone that shortens the lifetime of the photosensitive member occurs, because not only the environmental dependency is small, but also discharge is not made.
Also, in the contact charging using discharge, in order to obtain a desired charge potential Vs for the member to be charged, it is necessary to apply, to the charging member, a DC (d.c.)(direct current) bias (Vs+Vth) resulting from adding a discharge start voltage Vth (the supply voltage of the contact charging member when the member to be charged starts to be charged by applying a d.c. voltage to the contact charging member) to the desired charge potential Vs. However, in the charge injection charging, because the charge potential Vs substantially identical with the DC bias applied to the charging member is obtained, a power supply for charging can be reduced in the cost.
As the contact charging member in case of the charge injecting system, the magnetic brush charging member or the fur brush charging member is preferably employed from the viewpoints of the charge and contact stability.
The magnetic brush charging member includes a magnetic brush portion of charged magnetic carrier particles (hereinafter referred to as xe2x80x9ccharging carrierxe2x80x9d) having an electrical conductivity and magnetism, which are forcedly magnetically held by a bearing member also serving as a feeder electrode. The magnetic brush charging member brings the magnetic brush into contact with the member to be charged and supplies an electricity to the bearing member.
More specifically, the charging carriers are forcedly magnetically held directly on a magnet or on a sleeve including a magnet therein as a magnetic brush, and the magnetic brush portion is brought into contact with the member to be charged while the magnetic brush charging member is stopped or rotated. Also, a voltage is applied to the charging member, to thereby charge the member to be charged.
The fur brush charging member includes a brush portion (fur brush portion) of electrically conductive fibers which is born on a bearing member that also serves as a feeder electrode, and brings the brush portion of the electrically conductive fibers into contact with the member to be charged to supply an electricity to the bearing member.
In the charging member, the charge property is deteriorated when its fibers fall due to a long-time use or a long-time leaving, and the charge uniformity is liable to be uneven due to the restriction of the brush diameter or the like. On the other hand, in the magnetic brush charging member, such a phenomenon does not occur, thereby being capable of conducting charging uniformly and stably.
The injection charge using the magnetic brush, as shown in a model diagram of FIG. 15, can be considered to be equivalent to a series circuit consisting of a resistor R and a capacitor C in an image forming apparatus where a magnetic brush made up of charging carriers 202 which is born on a charging sleeve 201 of a magnetic brush charging device is brought into contact with a photosensitive drum 200 to charge the photosensitive drum 200. In an ideal charging process where injection charge is conducted due to the charging carriers 202, the capacitor C is charged in a time where the surface of the photosensitive drum 200 is anywhere in contact with the charging carriers 202 ((the width of a charged nip portion N)xc3x97(the peripheral speed of the photosensitive drum 200)), and the surface potential of the photosensitive drum 200 becomes substantially equal to the supply voltage.
(C) Cleanerless System (Cleanerless Process)
In recent years, in the image forming apparatus of the transfer system, a cleanerless system has been put into practical use in which remaining transfer toner that remains on the photosensitive drum is once collected by the above magnetic brush charging device for the purposes of not producing waste toner from the viewpoints of downsizing and simplification of the apparatus or ecology.
The toner that has been once collected by the charging device is brought into contact with the charging carriers so that charges equal in polarity to the potential of the photosensitive member are given to the toner, and the toner is discharged to the surface of the photosensitive member from the brush of the charging carriers due to an electric field developed by a potential difference xcex94V between the supply voltage and the surface potential of the photosensitive member. The toner discharged onto the photosensitive member is collected by the developing device again. The principle is shown in FIG. 16. The DC bias for development is set to be lower than the potential of the photosensitive member while the discharge of the toner is effected by the charging apparatus (charging device), and the toner discharged onto the photosensitive member due to the potential difference and the mechanical frictional force is collected to the developing apparatus (developing device).
However, the magnetic brush charging device using the above magnetic brush charging member suffers from a problem in that the charging carriers that form the magnetic brush may be stuck onto the surface of the image bearing member and flow out therefrom.
In particular, in case of the cleanerless system that collects the transfer remaining toner by the magnetic brush charging member (injection charger), the toner is mixed in the magnetic brush, and its electric resistance gradually increases. For that reason, the charges are not sufficiently moved while it passes through a charge nip, as a result of which the surface potential of the photosensitive member after the charges have passed through the charge nip becomes smaller than the supply voltage (the potential difference between the surface potential of the photosensitive member and the supply voltage is xcex94V). In the case where no means for detecting the surface potential or no means for controlling the developing bias is provided, the drop of the potential of the photosensitive member induces the adhesion of the toner onto a non-image portion during development, that is, so-called fog. On the other hand, in the case where the AV is large, the charging carriers of the magnetic brush is stuck onto the surface of the photosensitive member and flow out from the magnetic brush portion to cause the failure of charging. Simultaneously, the flow-out charging carriers are liable to be collected by the developing device.
In the developing device using an inductance detecting type sensor, when the charging carriers on the magnetic brush charging device side are collected by the developing device, even if the charging carriers of the slight amount that causes no problem from the viewpoint of an image are stuck onto the surface of the photosensitive member, the charging carriers are stored within the developing device as the number of copies becomes large. As a result, in the case where the permeability is different between the charging carriers and the developing carriers of the two-component developer, the apparent permeability of the entire developer varies, whereby there may occur an error in the toner density control by the inductance detecting type sensor.
In other words, in the case where the permeability of the charging carriers are larger than that of the developing carriers, although the toner density of the developer within the developer container is kept constant, if the charging carriers are mixed with the developer, the inductance detecting sensor detects that the average permeability of the developer is large. Since this means that the ratio of the carrier particles to the developer within the constant volume becomes larger, and the toner density becomes lower, the replenishment of toner starts and the density is so controlled as to be higher than an appropriate toner density.
On the contrary, in the case where the permeability of the charging carriers are smaller than that of the developing carriers, if the charging carriers are mixed with the developer, the inductance detecting sensor detects that the average permeability of the developer is small. Since this means that the ratio of the carrier particles to the developer within the constant volume becomes smaller, and the toner density becomes higher, the replenishment of toner stops and the density is so controlled as to be lower than an appropriate toner density.
The former case induces a problem in that the image density becomes higher due to the over-replenishment of toner, a problem in that the amount of developer increases with an increase in the amount of toner and the developer is leaked from the developer container, or a problem in that the toner is scattered due to the lowered toner charge amount with an increase in the ratio of the toner to the developer. Also, the latter case induces a problem in that the image is deteriorated due to a reduction in the amount of toner in the developer, the density of image is thinned or the density of image is thinned due to an increase in the toner charge amount.
Also, there is the possibility in that the adverse influence of the above problems is increased particularly according to the image forming operation, that is, as the number of copies is increased.
In addition, according to the study by the present inventors, there has been found that the adhesion of the charging carriers onto the photosensitive drum is promoted more particularly as the density of image is high or as the image ratio is large. For example, the mixing amounts of the charging carriers with the developing device, for example, when the solid images of A4 size and 0%, 6%, 15% and 30% in the image ratio are formed in 50,000 sheets were about 5 g, 7 g, 10 g and 15 g, respectively. It is presumed that as the image ratio is large or the image density is high, the transfer remaining toner collected by the charging device increases, and as mentioned above, AV increases due to the resistor of the charging carriers, and the mixing amount increases. It is also presumed that the reason why the mixing amount is not simply proportional to the image ratio is that the physical adhesion of the charging carriers onto the photosensitive drum surface or the carrier scattering due to the centrifugal force during the image forming operation does not depend on the image ratio but becomes uniform (see FIG. 11). In other words, in the above case, it is presumed that the mixing amount 5 g of charging carriers when the image ratio is 0% is the unavoidable mixing amount of charging carriers due to the influence of the physical adhesion of the charging carriers onto the image bearing member or the influence of the carrier scattering which is caused when the image has been formed in 50,000 sheets.
An object of the present invention is to provide an image forming apparatus which is capable of forming an excellent image even if electrically conductive particles provided in charging means enter developing means.
Another object of the present invention is to reduce an error occurring in the toner density control of an inductance detecting sensor in a developing device as much as possible even if the charging carriers on the magnetic brush charging device side are mixed with the two-component developer on the developing device side in an image forming apparatus of the magnetic brush contact charging system, the two-component developing system, the transfer system or the cleanerless system.
Still another object of the present invention is to provide an image forming apparatus, comprising: an image bearing member for bearing a latent image; charging means for charging the image bearing member by bringing ears of electrically conductive particles into contact with the image bearing member; developing means for visualizing the latent image formed on the image bearing member as a toner image by using a developer containing toner and carrier after the image bearing member has been charged by the charging means, the permeability of the carriers being different from the permeability of the electrically conductive particles; density control means for controlling the density of the developer within the developing means, the density control means detecting the permeability of the developer, comparing a detected signal value based on the detected result with a reference value, and controlling the density of the developer on the basis of the comparison result; and reference value correcting means for correcting the reference value, the reference value correcting means estimating the amount of electrically conductive particles that enter within the developing means and correcting the reference value on the basis of the estimated result.
Yet still another object of the present invention is to provide an image forming apparatus, comprising: an image bearing member for bearing a latent image; charging means for charging the image bearing member by bringing ears of electrically conductive particles into contact with the image bearing member; developing means for visualizing the latent image formed on the image bearing member as a toner image by using a developer containing toner and carrier after the image bearing member has been charged by the charging means, the permeability of the carriers being different from the permeability of the electrically conductive particles; density control means for controlling the density of the developer within the developing means, the density control means detecting the permeability of the developer, comparing a difference between a detected signal value based on the detected result and a reference value with a threshold value, and controlling the density of the developer on the basis of the comparison result; and threshold value correcting means for correcting the threshold value, the threshold value correcting means estimating the amount of electrically conductive particles that enter within the developing means and correcting the threshold value on the basis of the estimated result.
Other objects and features of the present invention will become more clear from the following detailed description with reference to the accompanying drawings.